This invention relates to a process for producing soybean milk giving a soft and pleasant feel to the tongue and having no undesirable beany or painty flavor or bitterness and, particularly, it relates to a process for producing soybean milk having such properties as described above, which comprises cooking soybeans in an aqueous medium, grinding the boiled soybeans into slurry while contacting them with an aqueous sodium bicarbonate, extracting the soyprotein and other water-soluble components from the ground soybeans and removing at least the major amount of solids from the slurry to give the desired soybean milk.
Some processes for preparing soybean milk have been reported. For example, U.S. Pat. No. 3,901,978 issued to A. I. Nelson et al. on Aug. 26, 1975 discloses a process for preparing a bland, stable aqueous dispersion of whole soybeans. However, the main purpose of such process is to reduce undesirable beany flavor or painty flavor, and the soft or pleasant feel given by the soybean milk to the tongue is not sufficiently improved.
Such process comprises adequately tenderizing soybeans by swelling and, heating them sufficiently to inactivate the lipoxidase enzyme contained therein, grinding them into a slurry, homogenizing the slurry and recovering an aqueous dispersion of whole soybeans. The resulting dispersion is relatively poor in soft and pleasant feel to the tongue because no solid particles are removed from the dispersion even if the slurry has been homogenized into a dispersion of finely divided particles dispersed in the aqueous medium.
U.S. Pat. No. 4,041,187 issued on Aug. 9, 1977, which is a continuation-in-part application of the above patent discloses a process for preparing an aqueous dispersion of soybeans which comprises heating soybeans sufficiently to inactivate lipoxidase enzyme contained therein, further tenderizing said soybeans to a predetermined degree and, after forming a slurry of the tenderized soybeans, homogenizing the slurry to obtain a soybean dispersion. The soybean dispersion prepared according to the process of the latter patent also feels relatively rough to the tongue because substantially no solids are removed and solid particles remain in the dispersion even if the slurry containing ground soybeans is homogenized. To promote the subsequent homogenization in the practice of such process, the soybeans must be sufficiently tenderized by boiling them for a long period of time. Although in this case the homogenization is effectively accomplished, the product still feels rough to the tongue because of the remaining solid soybean particles.
The latter U.S. Patent also discloses the cause of the undesirable flavor and odor as follows. The undesirable flavor and odor are principally caused by an enzyme system, lipoxidase, which catalyzes the oxidation of the polyunsaturated oils and fats in the soybeans. The reaction can take place quickly whenever: (1) the bean cell structure is damaged, as when the bean cotyledons are bruised, cracked or ground, so that the enzyme and oil are permitted to contact each other, and (2) a certain amount of water is present. As little as one part in one billion of the oxidized reaction product can be detected organoleptically and, for this reason, it is difficult and expensive to remove sufficient reaction product to impart a bland taste to the resulting beverage. Thus, it is extremely desirable to prevent the formation of such reaction product.
For these reasons, soybean milk produced by the prior art methods has been undesirable for use as a beverage or as beverage base and, therefore, it has been used as a raw material for other food products.
From their study and research relating to production of soybean milk having the above mentioned properties, they found the following facts:
1. Undesirable beany flavor can be removed by cooking the soybeans in an aqueous medium;
2. Even if the cooking time is not long enough to completely deactivate lipoxidase, the undesirable flavor can be completely removed by grinding the cooked soybeans while bringing them into contact with an aqueous medium at a temperature above 80.degree. C.;
3. If the cooking is effected in a short period of time, the insolubilization level of water soluble components, especially water soluble protein, can be significantly reduced;
4. Bitterness can be removed by grinding the cooked soybeans in the presence of sodium bicarbonate.
In accordance with these facts, the inventors continued their study and research and completed this invention.